By heating the soil samples to very high temperatures, and measuring the weight loss (loss on ignition) after burning at selected temperatures, we were able to determine which clay minerals maybe present in the samples and their relative amounts. Normally, kaolinite is dehydroxylated between 450°C and 550°C and losses about 13.95% its original weight. To carry out this experiment, we need a mass balance, drying oven, and crucibles. The following procedure is followed.
1. The crucible is weighed using the mass balance
4. The crucible + dried sample are weighed. The difference yields the water content
5. The oven is set at various temperatures ranging from 105 to 600°C and the crucible left in it for at least 1 hour at each temperature.
6. The crucible is removed from the oven and allowed to cool briefly.
7. Crucible + sample are re-weighed. The difference from the dry state yields the loss on ignition.
Based on the results obtained from all the samples, we were able to estimate the amount of kaolinite in each sample. From figure 16, the rate of weight loss on ignition between 450 - 550°C is very reasonable. This can be attributed to the dehydration of kaolinite. This tells us there is a reasonable amount of kaolinite in the FET sample. Observing figure 17, we have about a 6% loss in weight between 450 - 550°C which indicates that about 40% of the soil is made up of kaolinite. However, we observe that below 450°C, we have a loss of about 4%, which can be attributed to other clay minerals which should be swelling clays and colloidal materials.
0 2 4 6 8 10 12 14 105 150 200 250 300 350 400 450 500 550 600 temperature (oC) ra te o f w ei g h t l o ss
Figure 16, Incremental loss on ignition at various temperatures - FET
0 2 4 6 8 10 12 105 150 200 250 300 350 400 450 500 550 600 temperature (oC) To ta l w ei gh t l os s (% )
From figure 18, the rate of weight loss between 450 - 550°C can be attributed to the loss of water from kaolinite. This tells us there is a reasonable amount of kaolinite in the FBM sample. Observing figure 19, we have about a 6.2% loss in weight from 450 - 550°C telling us about 40- 45% of the soil is made up of kaolinite. This sample seems to be that with the least amount of swelling clays and/or colloidal material, since we observe from figure 19 that below 450°C, we have a loss on ignition slightly below 2%.
0 2 4 6 8 10 12 14 105 150 200 250 300 350 400 450 500 550 600 temperature (oC) ra te o f w ei gh t l os s
Figure 18, Incremental loss on ignition at various temperatures – FBM
0 2 4 6 8 10 105 150 200 250 300 350 400 450 500 550 600 tempeature (oC) To ta l l os s in w ei gh t ( % )
Figure 19, Total loss in weight at various temperatures – FBM
From figure 20, the rate of weight loss on ignition between 450°C and 550°C is quite much and also very outstanding. This loss corresponds to the dehydration of kaolnite, which tells us there is a reasonable amount of kaolinite in the FNK sample. Observing figure 21, we have about a 6% loss in weight from 450 - 550°C telling us about 40% of the soil is made up of kaolinite.
However, from figure 21, this sample also may be contents a reasonable amount of swelling clays and/or colloidal materials given the ±4% loss below 450°C.
0 2 4 6 8 10 12 14 105 150 200 250 300 350 400 450 500 550 600 temperature (oC) ra te o f w ei g ht lo ss
Figure 20, Incremental loss on ignition at various temperatures – FNK
0 2 4 6 8 10 12 105 150 200 250 300 350 400 450 500 550 600 temperature (oC) To ta l w ei gh t l os s (% )
Figure 21, Total loss in weight at various temperatures – FNK
From figure 22, we have a high rate of loss in weight upon ignition occurring between 450 - 550°C. This corresponds to loss of water from kaolnite, which tells us there is a reasonable amount of kaolinite in the FNB sample. Observing figure 23, we have about a 6% loss in weight from 450 - 550°C telling us about 40% of the soil is made up of kaolinite. This sample is also made up of a reasonable amount of maybe swelling clays and/or colloidal material given the ±5% loss below 450°C shown in figure 23.
0 2 4 6 8 10 12 14 105 150 200 250 300 350 400 450 500 550 600 temperature (oC) r at e o f w ei g h t lo ss
Figure 22, Incremental loss on ignition at various temperatures – FNB
0 2 4 6 8 10 12 14 105 150 200 250 300 350 400 450 500 550 600 temperature (oC) To ta l l os s in w ei gh t ( % )
Figure 23, Total loss in weight at various temperatures - FNB
From the loss on ignition results presented above, we can say all these samples most likely contain the amount of kaolinite which a soil should have in order to satisfy one of the most important selection criteria for the MIP technique [29]. However, we cannot be very certain and sure bearing in mind that dehydroxylation of other minerals especially amorphous silicates or organic matter between 450 - 550°C can mislead us. Sample FBM stands out as the best based on the fact that below 450°C, we observe only a ±2% loss on ignition while for the other 3, it is doubled or more. The other samples could therefore content reasonable amounts of undesired components.
Chapter-5
Methodology
5.1
Materials
The specimens for this study were fabricated using the following four components; kaolinitic soils from Cameroon (FBM, FET, FNB and FNK), 30% sodium hydroxide solution, distilled water and fine-grained sand. The soil samples were sun dried and sieved with only grain sizes below 425 m used for specimen fabrication, except for 2 series of specimens whose grain sizes were below 212 m and 106 m. The 30% Sodium hydroxide solution, fine-grained sand and distilled water were supplied by the laboratory.
5.2
Fabrication of Specimens
From equation (6), for a complete reaction between kaolinite and sodium hydroxide to take place, we need 2 moles of sodium hydroxide to react with 1 mole of Kaolinite. We started with an initial sodium hydroxide content of 8% weight of soil because we never have full conversion and at 8%, we are not far from equilibrium, taking into consideration the kaolinite content of the soils indicated from their loss on ignition. Water contents slightly below plastic limits were used for the first series of samples. This was to take care we do not use excessively high water contents , for this will result in non-homogenous mixtures and when the water evaporates empty cavities are left in the materials and this has negative effects on their strength and stability. Keeping the compositions of the other components constant, the water contents were first of all varied so that we could get the optimum water content for each sample. After obtaining the optimum water content, optimum contents for the other components were determined by keeping the amounts of the three other components constant while using variable amounts of the fourth component in each series.
To optimize the amount of sodium hydroxide, for each series, we also considered the fact that very low or very high amounts of sodium hydroxide will not give us good results. Very small amounts will not lead to the reaction of all the kaolinite present and so will lead to weak network polymers to bind the soil and sand grains together. Very high amounts of sodium hydroxide on the other hand may result in the material having un-reacted sodium hydroxide, which leads to
increase efflorescence (reaction between sodium hydroxide and carbondioxide to produce sodium carbonate precipitates), low stability and little durability. High amounts of sodium hydroxide will also unnecessarily increase cost.
5.2.1 Mixing
The mixer we used was the Small Hobart mixer. Components were all weighed separately and put into the mixer systematically. The sodium hydroxide solution was first weighed in a small plastic container. The required distilled water was then added to it and the mixture well shaked and closed. In some cases, water had to be subtracted from the sodium hydroxide solution. The required amount of solution was weighed and put in an oven at 105°C until the amount of excess water evaporates. The sand and soil were first put into the bowl of the mixer and mixed at speed 1 (~107 revolutions/minute) for about one minute. The sodium hydroxide solution/distilled water mixture was then added into the mixer while it was rotating at speed 1. Mixing continued in speed 1 for 2 minutes after which the mixer was stopped and the speed changed to speed 2 (~198 revolutions/minutes), in which the mixing lasted 10 minutes. In most cases, mixing was interrupted in order to remove samples stuck in the rotating arm or the walls and bottom of the bowl using a spoon. The mixture was also well inspected to see if it was dry or not (determine if the rheology is good).
5.2.2 Moulding
Moulding was carried out using a cylindrical steel mould. With the internal pressure of the oil at pressures of 60bar, compaction pressures of about 150bar (15MPa) were applied. After mixing, 10 specimens of approximately 50g each were moulded. This was done as fast as possible in order to avoid drying which will result in a decrease in workability. Specimens had various heights with an average about 45mm and diameters averagely 25mm. After moulding, the specimens are numbered, weighed and immediately put in an oven set at 80°C for curing. In order to check if there was swelling, the dimensions of one series of specimens were measured immediately after moulding. Comparing these dimensions with those after post curing we could determine if there was swelling or not.
5.2.3 Curing
Specimens were cured for about 24hours in an oven set at a constant temperature of 80°C. After curing, the specimens were removed from the oven and allowed to cool. They were again weighed and properly numbered and labeled. The samples were now ready for post curing, which was carried out under different conditions similar to those in which building materials are exposed to naturally.
5.2.4 Post curing and Pre-test Treatments
For each series of specimens, the first 3 (specimens 1, 2 and 3) were put in an oven set at a constant temperature of 40°C for at least 7 days. The next 3 (specimens 4, 5 and 6) were immersed in water for at least 7 days. The third group of 3 specimens (specimens 7, 8 and 9) underwent at least five cycles of drying and wetting, i.e. at least 5 days in the oven set at 40°C and at least 5 days immersed in water. This third group of specimens had to be immersed in water for at least 24 hours before the strength test. The last specimen (specimen 10), was half immersed in distilled water in order to check efflorescence. When partially immersed, water transports un-reacted chemicals and soluble substances from the immersed zones of specimens to the upper un-immersed surfaces. The deposited material was either whitish or reddish in color and in some cases grows into needle-like structures. This deposition of un-reacted chemicals and soluble substances on specimen surfaces is what is referred to as efflorescence. This is an indication of possible degradation as the un-reacted sodium hydroxide causes degradation of the material’s structure with time.